1. Field of the Invention
The present invention relates to a positioning system in a mobile communication system, and more particularly to a positioning system and a position calculating method in a mobile communication system for specifying a position of a mobile station on the basis of positional information from three base stations or on the basis of positional information initially from three base stations and subsequently from two base stations.
2. Description of the Related Art
At present, in a mobile communication system, there is a positioning system with a global positioning system (GPS) using an artificial satellite as a means for obtaining a position of a mobile station.
In a mobile communication system for the current personal digital cellular (PDC) or personal handy-phone system (PHS), there is a service for providing others with positional information by obtaining a position from a base station, but there is no system comprising a mobile station unit having a means for measuring its own position.
In a code division multiple access (CDMA) mobile communication system, however, there is a conventional technology in which a mobile station unit obtains its positional information on the basis of positional information from a base station.
This conventional technology is described in Positioning System” laid open as Unexamined Patent Publication (Kokai) No. 7-181242 on Jul. 21, 1995 (Applicant: Sony Corporation, Inventor: Takehiro Sugita).
This conventional technology is a CDMA digital mobile communication system in which a mobile equipment calculates a position of the mobile equipment with obtaining time differences at transmission of PN codes of four base stations by using coordinates of the four base stations and propagation time periods of signals to be transmitted.
In the conventional positioning system in the GPS, however, there is a problem that it requires a dedicated receiver and a dedicated antenna for receiving radio waves from an artificial satellite for a use of the GPS, which results in an expensive and large-sized apparatus.
In addition in the Positioning System” in the above Unexamined Patent Publication (Kokai) No. 7-181242, as described that Signals from at least three base stations need to be received in order to specify a location of the mobile station. The more the system has base stations from which signals can be received, the more an estimated precision is improved” in paragraph [0034] in the specification, the description is confined to the matter of specifying the position of the mobile equipment by obtaining the coordinates from the four base stations, but there is no description of a method for calculating the position of the mobile equipment by using only three base stations.
Therefore, the coordinates of the four base stations are used, which causes such problems that an infrastructure is heavy to calculate positional information of the mobile equipment and that a complicated calculation for evaluating the position increases a power consumption of the mobile equipment.
Furthermore, in the above Positioning System,” there is a description only of a transmission to the base stations or an administrative station or of a transfer to others as for processing after specifying the position of the mobile equipment, without taking into consideration an effective use of the calculated positional information of the mobile equipment by an owner.